The present invention relates to sliding components such as a swash plate and a piston in a compressor for a vehicle air-conditioning system.
A compressor includes some sliding components in its inner mechanism. Generally, gas (for example, refrigerant gas such as chlorofluorocarbon) circulates in the compressor in accordance with the operation of the compressor. The gas atomizes lubricating oil stored in the compressor and transfers the atomized lubricating oil to the sliding portions of the respective sliding components so as to lubricate the sliding portions between the sliding components. However, when the compressor is restarted after it has been left stopped for a long time, lubricating oil on the sliding portions is possibly washed away by refrigerant gas.
For example, in a swash plate type compressor, pistons are coupled with a swash plate through shoes and reciprocate in respective cylinder bores by the rotation of the swash plate or by the oscillation of the swash plate. At an initial operation of the compressor, the swash plate and the shoes slide on each other before lubricating oil reaches sliding surfaces between the swash plate and the shoes. Additionally, before the lubricating oil reaches the sliding surfaces between the swash plate and the shoes, gaseous refrigerant reaches the sliding surfaces and washes away the lubricating oil that was left on the sliding surfaces from prior operation of the compressor.
After the compressor has started, the refrigerant gas returns to the compressor and begins to atomize the lubricating oil after a certain period (approximately a minute). During this period, the sliding portions that need lubrication are insufficiently lubricated while the compressor is operating. Therefore, some conventional techniques have been proposed for ensuring lubrication on the sliding portions during this initial period.
Conventionally, the surfaces of the sliding components, such as a swash plate and a piston, are coated with resin containing solid lubricant for several reasons. One is for preventing seizure of the sliding surfaces. Another is for improving abrasion resistance. The other is for reducing friction coefficient. The above technique is, for example, proposed in Unexamined Japanese Patent Publication No. 11-13638. As disclosed in the above publication, column [0006], the base material of a swash plate is made of iron series or aluminum series. The surface of the swash plate (strictly, the sliding surface against a shoe) is coated with a coating layer, such as tin and copper. The coating layer is further coated with a sliding layer made of polyamideimide resin and a solid lubricant such as molybdenum disulfide and graphite.
To further ensure reliability, further improvement of certain characteristics, such as seizure resistance, abrasion resistance and adhesion to the base member are required. For example, carbon dioxide gas has been recently considered for as a possible refrigerant for a compressor. To compress carbon dioxide gas as the refrigerant, large compression load acts on a swash plate through the pistons in comparison to the compression load when a chlorofluorocarbon series is used as the refrigerant. The conventional coating layers of tin or copper are insufficient to achieve sliding performance in such a severe condition. The improvement of certain characteristics, such as seizure resistance, abrasion resistance and adhesion to the base member, contributes to the improvement of the sliding performance.